This invention relates to papermakers' fabrics and especially to papermaking fabrics for the press section of a papermaking machine.
In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a travelling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium the somewhat self-supporting paper web is transferred to the press section of the machine and onto a press fabric, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated, generally cylindrical rolls to remove still further amounts of water therefrom.
In general, the press fabrics in the press section are used in papermaking machinery to support the moist, freshly formed paper web as it encounters a variety of rolls to extract water from the moist paper web. In addition to serving as a support for the moist paper web, the press fabric serves as a receptacle for the water removed from the paper sheet. Preferably, the press fabric incorporates at least one base fabric woven so as to have relatively large open areas or voids which will enhance its water-conveying capabilities. To the base fabric is added one or more layers of batt material. The press fabric normally has a conveyer belt-like shape and during the various operations previously mentioned, a large amount of water built up in the press fabric, generally in the base fabric, is subsequently removed by suction or various other drainage devices, usually after the paper web and press fabric are no longer in direct contact.
In the press section of the machine, it is advantageous to remove as much water as possible from the paper web without causing any damage thereto, such as by crushing. Crushing occurs when an excessive amount of water is removed from the paper web, the excessive water then accumulating at the ingoing side of the nip, not carrying through the nip, and moving in a direction counter to that of the moving paper web. Accordingly, care should be taken to ensure that there is sufficient void volume present in the area of the press nip (either in the papermaking fabric or in the press rolls) such that as the paper web undergoes compaction and compression, all of the water expressed therefrom will be carried through the nip.
The resilience of a press fabric is calculated by the change in permeability and caliper of the press fabric from when it enters the press nip to when it leaves the press nip. The decrease in permeability and/or caliper in the press fabric after going through the press nip is used as a parameter for compaction resistance.
Moreover, characteristics such as paper sheet dewatering, sheet surface smoothness, sheet pick-up and sheet carrying, among others, are sensitive to press fabric design. An ideal press fabric will promote an increased area of interface, or contact area, between the fabric and paper, leading to greater adhesion between the fabric and the sheet, and therefore to improved sheet pick up and sheet carrying performance. In addition, the increased sheet/fabric interface area provides a more effective sheet support in the press, leading to more efficient sheet dewatering.
The surface of the press fabric is determinative of the marking characteristics found on the paper. The surface of the base fabric is usually determinative of the amount of shadow marking. If the surfaces of both the base fabric and the press fabric are more uniform, drainage of the press fabric will be better and the sheet surface quality will be higher. The quality of sheet surface uniformity is determined by the combination of contact points and contact area of the press felt, and in press fabrics having good uniformity, the contact area is increased.
Therefore, one object of the present invention is to provide an improved papermakers' fabric for use in the press section of the papermaking machine.
Another object of this invention is to provide a papermakers' fabric having means for increasing the amount of water removed from a paper web in the press section of a papermaking machine.
A further object of the present invention is to provide an improved papermakers' press fabric having increased compaction resistance and better resilience.
Still another object of the present invention is to provide an improved papermakers' press fabric providing superior wear resistance.
A further object of the present invention is to provide a press fabric having an improved surface, giving uniform drainage and better sheet quality.
Another object of the present invention is to provide a papermakers' press fabric with a stabilized batt layer, providing constant runnability.